Process of high reliability for communications between a master installation and secondary installations and device for carrying out this process

ABSTRACT

A process of transmission wherein each transmitted message is constituted by sequences containing element of information numerically coded by amplitude modulation of a carrier wave through at least one subcarrier oscillation. The coded values of each subcarrier oscillation are selected between two values close to a basic frequency of this subcarrier oscillation. The apparatus for carrying out this process includes in combination a number of means providing for a high reliability of the transmission.

United States Patent 1 Barret et al.

[ 51. Feb. 20, 1973 [54] PROCESS OF' HIGH RELIABILITY FOR COMMUNICATIONS BETWEEN A MASTER INSTALLATION AND SECONDARY INSTALLATIONS AND DEVICE FOR CARRYING OUT THIS PROCESS Inventors: Jean Pierre Barret, Marly 1e Roi; Gerard Jourdan, Paris; Jacques Harbonn, Jouars Ponchartrain, all of France [73] Institut Francais Du Petrole, Des

Carburants Et Lubriilants, Rueil Malmaison, Hauts de Seine, France Filed: April 2, 1970 Appl. No.: 25,017

Assignee:

{30] Foreign Application Priority Data April 3, 1969 France ..69l0366 U.S. Cl. ..340/5 R, 325/30, 340/16 C Int. Cl ..II041' 11/00 Field 01' Search ..340/5, 16 C, 171 A; 325/37,

DETECTOR FILTER [56] References Cited UNITED STATES PATENTS 3,348,226 10/1967 Fischer ..325/37 X 2,643,369 6/1953 Manley et al. 3,427,554 2/1969 Lagoe et al. 2,282,102 5/1942 Tunickm. 3,299,358 1/1967 Wood 3,313,160 4/1967 Goldman ..340/l6 C Primary Examiner-Richard A. Farley Att0mey-Craig, Antonelli, Stewart & Hill [57] ABSTRACT A process of transmission wherein each transmitted message is constituted by sequences containing element of information numerically coded by amplitude modulation of a carrier wave through at least one subcarrier oscillation. The coded values of each subcarrier oscillation are selected between two values close to a basic frequency of this subcarricr oscillation.

The apparatus for carrying out this process includes in combination a number of means providing for a high reliability of the transmission.

30 Claims, 9 Drawing Figures g FlLlER O FlLlER 2 i L0 95 DISCRIMIHATOR m '32 97 PROCESS OF HIGH RELIABILITY FOR COMMUNICATIONS BETWEEN A MASTER INSTALLATION AND SECONDARY INSTALLATIONS AND DEVICE FOR CARRYING OUT THIS PROCESS The present invention relates to a process of high reliability for communications between a main or master installation and one or several secondary or slave installations, and to a device for carrying out this process. In this process, the transmission of messages between the installations may be carried out in the form of signals flowing along an electric cable, or signals carried by electromagnetic or acoustic waves, or also by using successively signals of these different types all along the path between the installations.

This process is however of very special interest; because of the high reliability of transmission which can be achieved therewith, every time when communications through acoustic or electromagnetic signals must be established between two stations. There will be more particularly contemplated in the following, by way of non-limitative example only, communications through acoustic signals between a master installation and underwater secondary installations, the term acoustic signals having a broad meaning including as well the waves transmitted in the range of the audiofrequencies, ultra and infra-sonic waves, shock waves and, more generally, all elastic waves generated by vibrations in water.

There has already being proposed processes for establishing communications through acoustic signals between a master installation and underwater secondary installations. Some of the processes which are utilized for this purpose, are based on the frequency modulation of a carrier wave. In other processes the informations are transmitted in the form of pulses.

These prior processes are however not always satisfactory with regard to reliability. The frequency bandwidth necessary for the transmission is very affected through noises and interferences due to other transmissions. Moreover, when the informations are transmitted in the form of pulses, the transmissions may also be affected through reflections from the waterbottom or from mobile screens constituted, for example, by fish shoals, tracks of ships, submarines etc and thus the message picked up by the receiving installation is not identical with that transmitted from the transmittin g station.

For obvious reasons of safety, the secondary installation must not execute a received instruction which has not been correctly understood, or which is different from the instruction transmitted from the master installation.

Consequently every disturbance will result in the received instruction not being executed and therefore the secondary installation will only seldom execute an instruction, in order to comply with conditions of maximum safety.

This is a severe drawback when for example; precise instructions are to be executed with certainty and full reliability through an underwater installation.

An essential object of the present invention is accordingly to provide a new process permitting an increase in the reliability of the transmission, through acoustic signals, of informations and instructions between a master installation and a plurality of underwater secondary installations, in order to obviate the aforementioned drawbacks of the prior art processes and an apparatus for carrying out this new process.

The process according to the invention for transmitting messages constituted by signals, between two installations, is in particular characterized by the combination of the two following steps a. transmitting from one of said installations messages comprising at least one sequence of signals containing at least one element of information, this element of information being numerically coded by amplitude modulation of a carrier wave through at least one subcarrier oscillation, giving to the frequency of said subcarrier oscillation a coded value selected from at least two distinct values, close to the value of a basic frequency of said subcarrier oscillation, and

b. determining at the installation the value of the frequency of said subcarrier oscillation, this frequency being representative of the numerical value of said element of information.

According to an advantageous embodiment of the invention, permitting a limitation of the frequency bandwidth necessary to the transmission of the messages and thus preventing the transmission of being disturbed by noises, each sequence of a message from an installation is transmitted over a period of at least 1 second and preferably more than 1.5 second and the signals contained in this sequence are detected at another installation only after stationary conditions of reception of this sequence at this other installation have been established.

In order that the sequences which constitute a message having the above-indicated characteristics may be read at the receiving station in their order of transmission, it will be possible to include in each sequence of the message an element of information representative of a serial number of this sequence in the transmitted message, this serial number being preferably also defined by the value of the frequency of at least one subcarrier oscillation.

A device for carrying out the process according to the present invention is in particular characterized in that it comprises in combination means for transmitting, from one of the installations, messages including at least one sequence of signals which contains at least one element of information, means for generating a carrier wave and means for generating at least one subcarrier oscillation, means for amplitude modulation of said carrier wawe by said subcarrier oscillation, means for numerically coding said element of information, by giving the frequency of said subcarrier oscillation a coded value selected from at least two values close to a basic frequency of said subcarrier oscillation and means located at another installation for determining the value of the frequency of said subcarrier oscillation, this frequency being representative of the numerical value of said element of information.

According to an advantageous embodiment, the device for carrying out the process of the invention will include means for detecting the signals contained in each sequence exclusively after stationary conditions of reception of said sequence have been established.

The invention will be described more in detail and further advantages will appear more clearly from the following description of a non-limitative embodiment, with reference to the appended drawings wherein FIGS. 1A to 1D illustrate non-limitative examples of application of the invention to the remote control of an underwater oil installation, such as an underwater oil wellhead, from a master installation.

FIG. 2 A diagrammatically illustrates the part of the receiving device equipping an underwater wellhead, which analyzes the messages received from the main installation.

FIG. 2 E diagrammatically illustrates the part of the device equipping the wellhead, which provides for the execution of the instructions received from the master installation and the means for transmitting messages from the wellhead.

FIG. 3 shows diagrammatically the constitution of a call message followed by an address message transmitted from the master installation.

FIG. 4 represents diagrammatically the constitution of a ready-to-operate message transmitted from the wellhead, in response to the address message to this wellhead.

FIG. 5 shows diagrammatically the transmitterreceiver device equipping the master installation.

In the application of the invention shown in FIG. 1 A by way of example, the master installation P is located on a floating structure 101 positioned above an underwater wellhead 102 equipped with a secondary installation T, adapted to control elements of the wellhead, at the reception of messages constituted by acoustic signals emanating from the master installation.

In the embodiment of FIG. 1 B, the master installation P is located onboard a control ship 103, wherefrom are controlled the elements of the wellheads of a producing oil field, through submerged secondary installations T .T, .T,

FIGS. 1 C and 1 D illustrate other embodiments of the system illustrated in FIG. 1B. FIG. 1 C corresponds to the case where the oil field is located not very far away from the shore 105 whereonis placed the master installation P and FIG. 1 D corresponds to the case where the oil field is located far away from the shore and the main installation equips a platform 104 fixed on the water bottom.

It will be possible without departing from the scope of the present invention to adapt to particular conditions of use the embodiment described hereinunder of a device according to the present invention, by employing means very well known to those skilled in the art.

It will be, for example, possible, in the embodiments of FIGS. 1 A, l C and I D, to use directional hydrophones for transmitting and receiving the acoustic signals at the different installations and moreover, in the embodiments of FIGS. 1 C and l D, it will be possible to use a device for adjusting the orientation of the hydrophone 1 equipping the main installation.

It is assumed in the following that conversations must be established by transmitting messages constituted by acoustic signals between the master installation P and each of the underwater installations, such as T taken individually, ordering to these installations to control a plurality of regulating members, such as valves, and

make these installations transmit informations comprising for example the position of valves, pressure measurements etc The instructions emanating from the master installation must be executed with the maximum reliability.

In the course of experiments the following conditions of use have been set forth.

a. the master installation must be able to converse selectively with any of the wellheads and with only one at each time, through the intermediary of a secondary installation T, associated with this wellhead. For this purpose a characteristic address is assigned to each wellhead;

b. for reasons of power saving, so as to increase the working life, in the case where the secondary installations are supplied from submerged power sources, such as batteries, each wellhead must have a permanent watching condition or state of reduced activity for listening whether any message is transmitted.

The value of the frequency F of the acoustic carrier wave has been chosen equal to 30 kHz for the transmissions both from the master installation and from the secondary installations, in view of the following requirements,

c. maximum transmission distance of 5 km for an electric power of watts of the transducer which, on the master installation, produces acoustic signals in response to electric control signals;

d. the transmission must not be disturbed by external acoustic signals.

For reasons of reliability the transmission must not be perturbated through parasitic signals, such as noises, so that a transmission through coded messages is preferred.

The number and the values of the basic subcarrier frequencies which can be used for coding the informations are limited on the one hand by the choice of the value of the carrier frequency, fixed at 30 kHz and, on the other hand, by the need of reducing the frequency bandwidth which is used, in order to decrease the sensitivity to noises.

In each message the elements of information have been coded in a binary form, by means of the frequencies of the modulating subcarrier oscillations, using a plurality of basic subcarrier oscillations and for each of these oscillations two values of the frequency, symmetrical with respect to the frequency of the basic oscillation, i.e., a minimum frequency which represents the value zero of the element of information, and a maximum frequency representing the value 1, in such a way that these two values cannot be present at the same time.

During the experiments four basic subcarrier frequencies have been selected F F F and F respectively equal to 400 Hz, 560 Hz, 730 Hz and 960 Hz so as to transmit, by means of the above-indicated binary code, eight elements of information, only the pressure measurements being transmitted in the form of analogical signals.

The above frequencies are standard frequencies (IRIG system) which have been selected so as to make impossible any confusion between-these frequencies of intermodulation of these frequencies.

The messages are transmitted from the master station to the wellheads according to the following pattern:

a. transmission of a call message followed with an address message corresponding to the wellhead with which the master station is willing to converse,

b. after the connection has been established between the master station and thewellhead, order or instruction signals are transmitted. These order signals may consist of instructions to be executed (opening or closing of valves, resetting to a condition of reduced activity), or interrogations regarding positions characterizing the condition of the wellhead.

FIG. 3 shows a call message followed with an address message for a wellhead, transmitted from the master station.

The call message 12 has in this embodiment a duration t, of 3 seconds. It is constituted by the pure carrier wave at the frequency F and is characterized by the absence of all of the modulating subcarrier freqencies F,, F,, F and F The address message 13 represents the address of the wellhead with which it is desired to established a conversation.

This address consists of four sequences 13 a, 13 b, 13 c and 13 d, having the same duration t equal, for example, to two seconds. Each sequence-is characterized in that it contains the carrier wave at the frequency F, amplitude modulated simultaneously by the four subcarrier frequencies F to F; all of the same amplitude while the frequency of each oscillation may take, as already indibated, any of the two values symmetrical with respect to the basic frequency.

In each sequence, the values of the frequencies F and F, are characteristic of the address and the frequencies F and F, are characteristic of the order of transmission of the sequences, so that at their reception the sequences can be read in the same order as at their transmission.

By-constituting the message with sequences each of a duration t greater than 1 second, it is possible to reduce the bandwidth necessary for the transmission of the messages and thus to prevent this transmission of being disturbed by noises, the signals contained in each sequence being detected at a receiving station only after a stationary condition of reception of this sequence, is established, for example at the instants indicated E on each of the sequences 13 a to 13 d and separated from the beginning of each sequence by a time interval t; (for example equal to 1.5 second).

FIG. 2 A shows diagrammatically the receiving part of the apparatus equipping an underwater wellhead, which detects the messages received from the master station.

In this embodiment wherein the master installation is stationary, the hydrophone l of this installation (FIG. 1 D) is directed substantially toward the selected wellhead T,.

A call message transmitted from the master station P is picked up by this wellhead and by the adjacent wellheads.

The message picked up by the hydrophone 2 (FIG. 2 A) is transmitted through the transmission-reception switching device 3 and the cable 131 to the receiver 4 whose sensitivity is preadjusted by the sensitivity switch 5. When the working conditions are known, the preadjustement of the switch 5 is effected before immersion of the secondary installation T During the operation, an automatic gain control device, of a type well-known to those skilled in the art, incorporated in the receiver 4, adjusts the sensitivity as a function of the signals picked up by the hydrophone 2 after demodulation at 6, the signal is transmitted to the filters 7, 8,9 and 10 through cables 47 to 50 respectively and to the watching device 11 through cable 46. The filters 7 to 10 are tuned to the basic values of the subcarrier frequencies F, to F respectively. The call message 12 (FIG. 3) received by the watching device 11 is only identified as such if it contains none of the four modulating subcarrier frequencies F, to F The absence of these frequencies is detected at the output of the filters 7 to 10, through the detecting devices 14 to 17 respectively, which deliver corresponding signals to the device 11 through cables 51 to 54 respectively. The device 11 is of the AND gate type, adapted to validate the call message only when signals are simultaneously received through cable 46 and cables 51 to 54.

When the validation of the call signal has been effected during a time interval greater than t t being for example equal to 2 seconds, the device 11 delivers a signal. This signal, transmitted through cable 55 to the memory 26, places the latter in a state of reception and actuates a retarding device (which may be a part of the device 11) whose retarding time t has for example been set at 11 seconds, so as to permit the analyzing of the following message.

This retarding device will put the secondary installation back to its condition of reduced activity, for example by interrupting the electric supply of the installation, except for the part thereof which must receive the call messages, if the remainder of the message, constituting the address message 13 (FIG. 3) is not identical with the address of the wellhead.

For the four sequences 13 a to 13 d representing the address, each of the basic modulating subcarrier frequencies F to F is selected by the corresponding filter 7, 8, 9 or 10 and transmitted to the discriminators 18 to 21, which may be of a known type, through cables 56 to 59 respectively The use of discriminators makes it possible to decrease the sensitivity to noises or signals whose frequencies are close to that of the signal which must be detected.

As hereinabove indicated, the informations transmitted by each basic modulating subcarrier frequency are numerically coded according to a binary code, using two frequencies whose values are symmetrical with respect to the value of the basic subcarrier frequency, i.e., a minimum frequency representing the value zero of the element of information represented by this modulating frequency and a maximum frequency representing the value 1 (the frequency interval between the maximum frequency and the minimum frequency being for example 60 Hz for each of the subcarrier frequencies).

Each discriminator supplies a signal at one of its output terminals, in relation with the value of the received modulating subcarrier frequency: the signal appears at a first of these terminals if this frequency has its maximum value or at the second terminal if the value of this frequency is minimum, since, as indicated hereinabove, the maximum value of each modulating subcarrier frequency and its minimum value, cannot be found at the same time.

The signals which, in the different received sequences, characterize the numerical values of the elements of information contained in these sequences, are supplied by the discriminators 20 and 21 corresponding respectively to the basic modulating subcarrier frequencies F and F and are transmitted through cables 64 to 67 to the memory 26 wherein they are recorded after validation of the sequence to which they belong, this validation being effected by the device 22 as described hereinunder.

The signals characterizing the serial number of the sequence, which are generated by the discriminators 18 and 19 corresponding to the basic modulating subcarrier frequencies F and F are transmitted through cables 60 to 63 to the device 23 for decoding the serial number, which may be of a known type. After decoding, the information characterizing the serial number of the sequence are transmitted through cables 77 to 80 to the memory 26, wherein they are recorded after validation of the sequence by the device 22.

The delivering, at each of the two pairs (77, 78; 79,80) of output terminals of the device 23, of signals characterizing the serial number of the analyzed sequence is indicated to the sampling device 106 by a device of the type OR inclusive gate incorporated to the device 23 through cable 109 and causes, through a derivation with respect to time, the generation by the device 106 of a sampling pulse delivered through a retarding device, when a time interval has elapsed from the beginning of the sequence. This pulse, transmitted to the memory 26 through cable 107, a gate 25 and cable 108, controls the recording of the informations emanating from the discriminators 18 to 21 through bistable devices of the memory 26, whereby the informations delivered by the discriminators 20 and 21 are recorded in this memory as a function of the informations emanating from the discriminators 18 and 19 characterizing the serial number of the sequence.

The sampling signal generated by the device 106 cannot control the recording of the informations contained in the analyzed sequence unless the gate 25 is opened. This gate is controlled by a safety device 22 which has an essential function in the operation of the whole device. It receives through cables 68 to 75 the signals delivered at each of the output terminals of discriminators 18 to 21. It ascertains, over the whole duration of the analysis of each sequence, first that each discriminator delivers a signal (using for example a gate AND), thus making sure that all the modulating subcarrier frequencies are present and secondly that, during each sequence and specially at the instant of the sampling, each discriminator delivers an output signal at only one of its output terminals for the above indicated reasons (using for example circuits of the type OR exclusive gate).

After these checking operations have been performed, the safety device 22 validates the sequence by delivering a signal which is transmitted through cable 76 and opens gate 25, thus making it possible for the sampling signal emanating from the device 106 to actuate the recording of the information contained in the the informations contained in the sequence, the safety device 22 does not actuate the gate 25 and consequently the informations are not recorded in memory 26.

The above-described sampling is allowed only during the receiving periods of the underwater installations. To this end, the sampling device 106 receives, through cable 114 connected to the transmitting means of the installation, a signal which precludes any sampling during the periods of transmission of messages from the underwater installation and also during a given time interval after the transmission. In that way, signals which may be produced in the receiver during the transmission periods, or those resulting from parasitic reflections due to obstacles present in the vicinity of the underwater installations, for example, are not analyzed through the underwater installation.

The sampling signal is also used for actuating a device 24 for automatically setting the installation into a condition of reduced activity, whose operation is described below.

The four sequences 13 a to 13 d (FIG. 3) constituting the address message have been analyzed and recorded in the memory 26, as explained hereinabove, the address received by the underwater installation is transmitted to the device 28 for decoding the address. If the received address is not identical with that assigned to the wellhead or, this being equivalent, if this address is erroneous, for example due to the lack of one sequence, the device 28 for decoding the address does not deliver any signal and the retarding device forming part of the watching device 11 resets the secondary installation to its condition of reduced activity, after a time interval i has elapsed from the beginning of reception of the address, for example by interrupting the power supply of this installation.

When the received address is identical with the one assigned to the wellhead, the device 28 (FIG. 2 B) delivers, on the one hand, a signal to the energizing device 30 through cable 82 and, on the other hand, locks through cable 133 the delay device in the watching device 11 thus preventing it to reset the underwater installation to its position of reduced activity.

The energizing device 30 energizes through cable 83 the control device 31 of the valves or other regulating members of the wellhead, i.e., places this control device into a condition wherein it is able to execute the instructions which will be received subsequently and actuates through cable 111, the device 34 controlling the emission.

The underwater installation thus energized transmits then to the master station a message indicating that it is ready to operate, such a message being shown diagrammatically in FIG. 4. The first part of this message is constituted by the address, as indicated hereinabove. The second part comprises two sequences, 36 and 37 successively, which characterize the condition of the wellhead.

The sequence 36 indicates in analogical form an information such as a pressure measurement in the wellhead. This sequence is characterized in that it comprises the carrier wave at the frequency F, amplitude modulated by a single subcarrier oscillation of constant amplitude, for example that of frequency F any other frequency being excluded.

This subcarrier oscillation is linearly frequency modulated between two predetermined values of the frequency, as a function of the transmitted information.

The sequence 37 indicates the position of regulating members of the wellhead, such as valves. This position is indicated with a binary code, using the four basic subcarrier frequencies F to F, simultaneously, according to a predetermined code.

In the case where the number of regulating members of the wellhead to be controlled is at most equal to the number of basic subcarrier frequencies it will be for example possible to assign to each regulating member one of these subcarrier frequencies whose minimum value may represent a first condition of this member, for example the closed condition, if this regulating member is a valve, while the maximum value may represent its open condition.

As illustrated by FIG. 2 B, the ready-to-operate message can be transmitted as soon as the energizing device 30 has been actuated, this device then delivering through cable 111 a signal energizing the device 34 which controls the emission from the secondary installation. V

The address of the energized wellhead, displayed on the device 28 is transmitted through cables 115 to 122 to the device 33 for coding the address, wherein this address is coded, this device being for example of a known type.

The pressure in the wellhead is measured in the form of an electric voltage through a device which may be of a conventional type and is not illustrated in the drawing. This voltage is supplied at 84 to the voltagefrequency transducing device 32, which generates an oscillating signal, whose frequency varies linearly as a function of the pressure.

In the contemplated embodiment, this frequency may vary between the values F 25 Hz and F, +25 Hz, the so-defined frequency interval of 50 Hz being so selected in order to make easy the numerical displaying of the so-measured pressure.

The address of the wellhead, the pressure measurement and the information concerning the position of the valves being transmitted to the device 34 through the cables 85, 86 and 87 respectively, this device elaborates the ready-to-operatemessage, as diagrammatically illustrated in FIG. 4.

This message is transmitted through cable 88 to th transmitter 35 connected through cable 132 to the transmission-reception switching device 3, and transmitted to the master installation through hydrophone 2.

When the ready-to-operate message has been received at the master installation P it is possible from the latter to order the secondary installation to execute instructions. These instructions may be given for different purposes a. to have executed by the secondary installation, the actuation of elements of the wellhead, for example opening or closing of valves,

b. be informed of the actual condition of the wellhead,

c. to reset the underwater installation to its condition of reduced activity.

The wellhead which is in communication with the master installation having being identified through the address message and the ready-to-operate message and its device 31 for controlling the valves (FIG. 2 B) being from now on energized, the subsequent messages of instructions are not preceded by the address and comprise only one sequence of a duration of two seconds during which the 'carrier wave is amplitude modulated by the four subcarrier frequencies F to F simultaneously.

For example, the instruction for opening valve Nr 1 may be coded as follows, using the four subcarrier frequencies 1 min 2 min) 3 min, 4 min,

(the indexes min and max correspond respectively to the minimum and to the maximum values of the fr equence of each subcarrier oscillation), while the instruction for closing the valve may be represented by l mar, 2 mlm 3 min, 4 min For valve Nr 2 the instructions may be coded as follows p g l min, F2 min, F3 mar F4 min, closing 1 F1 max F2 mar F3 min, F4 min and so on for the following valves.

An interrogation may for example be represented by the code 1 mar 2 maxi 3 max: 4 max,

and an instruction for resetting to the condition of reduced activity by the code 1 min, 2 mtm 3 mtm 4 min In the indicated example, the number of basic subcarrier frequencies being limited to four, it is possible to transmit 16 different coded messages, and since two of these messages must be an interrogation message and a message for resetting the installation to a condition of reduced activity, this means that fourteen messages of instructions can be transmitted.

At the receiving of a message containing an instruction, the latter is transmitted to the discriminators 18 to 21.

The discriminators 18 and 19 transmit their signals to memory 26 through cables 123 to 126, while the discriminators 20 and 21 transmit their signals to the memory 26 through cables 64 to 67. These signals are recorded by the memory after validation of the sequence by the device 22, as above-indicated in connection with the address message. The signals delivered by the discriminators 18 and 19 are furthermore transmitted to the decoding device 23 to permit the elabora tion of the sampling pulse, as previously indicated, but are not taken into consideration for the transcription of the instruction message into the memory. Only the signals which are transmitted to memory 26 through cables 64 to 67 and 123 to 126 record in the memory 26 the instruction received from the master installation.

In the case of a message containing instructions to actuate regulating members, such as valves, after the message has been recorded in memory 26, it is transmitted (FIG. 2 B) through cable 89 to the device 31 for controlling the valve, which device has been previously energized by the address message.

This device 31 then actuates a device for performing the desired operations on the valves.

In the case of an interrogation message, this message is transmitted from memory 26 to the device 29 for decoding the interrogation, through cables 127 to 130, the device 29 then delivering a signal transmitted through cable 90 to the device 34 which elaborates a message in response to the interrogation and controls the transmission of this message from the secondary installation. This message is constituted by two sequences identical to the above-described sequences 36 and 37 and indicating respectively the pressure value and the position of the valves.

The informations which are required to elaborate this message are supplied to the device 34 in the same manner as for the ready-to-operate message.

In the case of an instruction of resetting to the condition of reduced activity, the message transmitted from memory 26 through cables 131 to 134 is decoded by the device 27 for decoding such messages. The device 27 delivers through cable 91 a signal which unlocks the retarding means of the watching device 11, previously locked when the installation has been placed in condition of conversation with the master station.

The device 27 also delivers a signal which, through cable 112, actuates the device 34, thus causing the transmission of a message indicating the return of the installation to its condition of reduced activity. This message is for example constituted by transmitting during two seconds the carrier wave simultaneously modulated by the four subcarrier oscillations having their minimum frequency value. The master station cannot start to converse with another underwater installation before it has received the response message indicating that the preceding underwater installation with which this master installation was conversing has returned to its condition of reduced activity.

The device 24 for automatic setting to the condition of reduced activity (FIG. 2 A) is an important safety element.

If a preselected time interval t has elapsed after the receiving of a message other than an instruction to return to the condition of reduced activity and no further message has been received by the secondary installation which is energized or is conversing with the master installation, the device 24 actuates automatically the resetting of the station to its condition of reduced activity. This device comprises, for example, a capacitor connected to a charging circuit having a charging time, necessary to reach a critical voltage, equal to t (t 2 minutes, for example). This circuit is actuated by the sampling pulse emanating from 106, which, through cable 110, discharges the capacitor every time the analyzed sequence is validated through the safety device 22 which opens gate 25.

Thus, for sequences which are not validated by the device 22, no pulse is produced for discharging the capacitor of device 24.

When the capacitance reaches the critical voltage, at the end of the time interval 1 the device 24 transmits to memory 26 the instruction for resetting to the condition of reduced activity, through cable 81.

Everything proceeds then as after the receiving of a message of resetting to the condition of reduced activity emanating from the master installation.

FIG. shows diagrammatically the apparatus on the master installation at the surface, this apparatus being for a great part similar to that at an underwater installation.

Like devices, performing the same operation, have been indicated by the same reference numerals as those used in FIGS. 2 A and 2 B, but marked with the index a.

The device 45 controlling the emission of the call and address messages acts upon the device 33 a for decoding the address. The coded address of the well head with which it is desired to converse is transmitted through cable 92 to the device 34 a, controlling the emission, which then elaborates a call followed with an address message of the type shown by FIG. 3, while through a mechanical connection 113 the address of the called wellhead is recorded in the device 28 a for decoding the address.

After reception of the first part of the ready-tooperate message reproducing the address of the called wellhead, the signalling device 38 displays the identity of the energized underwater installation after its address has been decoded in device 28 a.

The second part of the ready-to-operate constituted by the sequences 36 and 37 (FIG. 4) is identified by the device 40 detecting the presence of the analogical information. This device 40, which is of the AND gate type simultaneously ascertains the absence of the modulating subcarrier frequencies F F F and the presence of the subcarrier frequency F carrying the analogical information.

This makes it possible to detect the beginning of the sequence 36 (FIG. 4) transmitting an analogical information, this sequence being always followed by a sequence 37 indicating the position of the valves.

These data concerning the subcarrier frequencies are supplied to the device 40 by the discriminators 19a, 20a and 21 a, through cable 92, and by the device 14 a detecting the subcarrier frequency F connected to the device 40 through cable 93. The device 40 then delivers to the device 43 through cable 95 a signal validating the received information.

This signal validating the indication is delivered both to the device 42 for reading the analogical information and to the device 44 for remotes indication of the positions of the valves of the called wellhead, so that this device 44 be ready to display the information concerning the position of the valves, which will follow the analogical measurement.

The analogical measurement received through cable 94, decoded at 41 by frequency demodulation, can thus be read on device 42, while the informations of position, stored in 26 a, are transmitted to device 44 through cable 97.

When the called wellhead has answered, the calling device 45 is locked through any known means, for example through mechanical means, so that no other call can be sent as long as the conversation with a wellhead is maintained. The device 39 makes it possible to elaborate the coding of the instructions which must be executed through remote control from the master installation, this coding being effected in the above-indicated manner.

As shown in the foregoing description, each type of message is characterized by its constitution, i.e., by its number of sequences and/or by the number of modulating subcarrier frequencies contained therein, this number being variable between zero and four in the embodiment of the invention which has been described as a non-limitative example.

The displaying devices of the apparatus which equips the master installation makes it possible for an operator to know at any time the position of the underwater installation with which the conversation is established, while the other wellheads are necessarily in their condition of reduced activity.

In the case where the number of regulating members on the underwater installations is very large it will be possible, instead of having messages of instructions and the response messages constituted by only one sequence of position, such as sequence 37 (FIG. 4), either to constitute these messages in a similar manner as for the address message illustrated at 13 (FIG. 3), whereby it is possible to transmit a great numberof informations with a minimum number of frequencies, or alternatively to transmit sequences indicating the position of regulating members, such as valves, separated by sequences giving analogical informations, i.e., repeat the sequences containing an analogical measure ment, such as 36, and the sequences of remote position determination, such as 37 (FIG. 4), these sequences corresponding for example successively to the different regulating members, according to a predetermined serial order thereof.

What we claim is l. A process for transmitting messages consisting of signals between two installations comprising the combination of the following steps:

a. transmitting from one of said installations messages comprising at least one sequence of signals containing at least one element of information, the element of information being numerically coded through the frequency of at least one subcarrier oscillation which amplitude modulates a carrier wave, the frequency having a coded value selected from at least two values close to a basic frequency of said subcarrier oscillation, and

b. determining at the other installation the value of the frequency of said subcarrier frequency, the determined frequency being representative of the numerical value of said element of information.

2. A .process according to claim 1, wherein said sequence of signals is transmitted over a period of at least one second and the signals contained in said sequence are detected at the other installation exclusively after establishment of stationary conditions of reception of said sequence.

3. A process according to claim 1, wherein each sequence of signals includes an element of information representative of a serial number of said sequence in the transmitted message.

4. A process according to claim 1, wherein the transmission of said messages between said installations is effected through at least one transmission cable.

5. A process according to claim 1, wherein said messages are transmitted between said installations by means of electromagnetic waves.

6. A process according to claim 1, wherein said messages are transmitted by means of acoustic waves.

7. A process according to claim 1, wherein said messages are made distinctive according to their nature by the number of sequences assigned to each message.

8. A process according to claim 1, wherein one of said installations effects the setting into a state of reception of another installation by transmitting a call message having a single frequency, this signal constituting a carrier wave and said signal is identified at said other installation by the presence of said single frequency exclusively in said signal.

9. A process according to claim 8, wherein at the receiving of said call message said other installation is kept in a state of reception during a determined time interval for analyzing a message following said call message.

10. A process according to claim 1, wherein for the transmission of messages between a master station and a plurality of secondary installations, the master station places at least one secondary installation into a stage of reception by transmitting a call signal thereto and following said call signal, the master station transmits sequences of coded informations, the group of said sequences constituting an address characteristic of said secondary installation, the informations contained in the different sequences being represented by a group of different modulating subcarrier frequencies, said sequences being decoded at said secondary installations and wherein is set in condition of conversation with the master installation exclusively, the secondary installation having an address which corresponds to said address following said call signal.

11. A process according to claim 1, wherein said sequences are constituted by a group of different basic subcarrier frequencies, wherein at least one of said sub carrier frequencies represents, according to a binary code, the value of an information, and at least one other of said subcarrier frequencies represents a serial number of .said sequence, so that two sequences differ at least by the value of at least one of said subcarrier frequencies.

12. A process according to claim 11, wherein at the reception each of said sequences is validated only after the presence of all of said subcarrier frequencies has been ascertained and after the element of information represented by each of said subcarrier frequencies has been verified, and a sampling of the signals contained in said sequence is effected after validation of said sequence.

13. A process according to claim 10, wherein a ready-to-operate" response-message is transmitted from said secondary installation, in response to the setting of said secondary installation into a condition of conversation, said response-message comprising the address of said installation.

14. A process according to claim 13 wherein after reception of said response-message at said master installation, said master installation transmits coded instructions representing operations to be carried out on at least one regulating member of said secondary installation, said instructions being transmitted to means for actuating said regulating member arranged on said secondary installation, exclusively when said secondary installation is in a condition of conversation, the instruction being coded with the frequency of the carrier wave and at the same time with all the different subcarrier frequencies of said group of subcarrier frequencies.

15. A process according to claim 14, wherein at the reception of an instruction a response-message is transmitted from said secondary installation, said response message being characteristic for the received instruction.

16. A process according to claim 10, wherein at the setting of said secondary installation in condition of reception a device is actuated for automatically resetting said secondary installation into a condition of reduced activity when a determined time has elapsed after the reception of each sequence of a received message other than an instruction from the master station of returning to the condition of reduced activity, and wherein said automatic device is set back to its initial position exclusively upon receiving of a new message of the conversation when this message is received during said time interval.

17. A device for transmitting messages consisting of signals between two installations, comprising in combination means for transmitting, from one of said installations, messages comprising at least one sequence of signals which contains at least one element of information, means for producing a carrier wave, means for producing at least one subcarrier oscillation, means for amplitude modulation of said carrier wave by saidsubcarrier oscillation, means for numerically coding said element of information, adapted to give the frequency of said subcarrier oscillation a coded frequency selected from at least two values close to a basic frequency of said subcarrier frequency and means determining at the other installation the value of the frequency of said subcarrier frequency, said frequency being representative of the numerical value of said element of information.

18. An apparatus according to claim 17 comprising at a first of said installations, means for transmitting messages and at a second installation, means for receiving these messages, said receiving means including a retarding device adapted to place said second installation in a state of reception during a fixed time interval, at the receiving of a characteristic call message, said time interval being at least equal to the duration of a message of information to be transmitted to said second installation after said call message, means at said second installation for validating said message -of information, said validating means being adapted to keep said second installation in a condition of conversation with said first installation by blocking said retarding device exclusively when said message of information is validated.

19. An apparatus according to claim 18, wherein said second installation includes means for placing automatically said second installation back into a condition of reduced activity at the end of a predetermined time interval after the receiving of each message of information and in the absence of the reception of new message of information and in the absence of the reception of new messages emanating from said first installation.

20. An apparatus according to claim 17, wherein each of said installations includes means for transmitting sequences of acoustic signals, said sequences having a duration greater than one second and means, in combination with said transmitting means, for receiving said sequences of signals. condition of reception of said sequence has been established.

21. An apparatus according to claim 17 wherein at said second installation includes means for controlling the transmission of a response-message containing the address of said second installation and addressed to said first installation.

22. An apparatus according to claim 18, wherein said first installation comprises means for displaying the informations received from said second installation, in combination with means adapted to validate the sequences of said message containing informations, prior to the displaying of these informations.

23. An apparatus according to claim 17, wherein at said second installation includes means for sampling signals of said sequences.

24. An apparatus according to claim 23, wherein said second installation includes means for blocking the operation of said sampling means during the transmission periods of said second installation and during a fixed time interval following each transmission period.

25. An apparatus for transmitting from a first installation messages destined to a second installation which is in a state of reduced activity, said messages comprising call and address messages for said second installation, so as to place in state of reception and energize said second installation at the receiving of said call and address messages and for transmitting from said second installation, in response to a call and address message a ready-to-operate response-message destined to said first installation, this apparatus comprising in combination:

a. at said first installation, means for transmitting a call message constituted by a signal at a call frequency, means for transmitting messages including at least an address message corresponding to said second installation and following said call message, said address message being constituted by at least one sequence of informations coded numerically by the value of the frequency of at least one subcarrier oscillation amplitude modulating a carrier wave, this coding frequency being selected between at least two values close to a basic frequency of said subcarrier oscillation, each sequence of the address message also including at least one information signal representative of a serial number of said sequence in said address message, and means for receiving a ready-tooperate response-message transmitted from said second installation, in response to said address message;

b. at said second installation, means for receiving messages emanating from said first installation and means for identifying said call message, adapted to deliver a validating signal at the receiving of a message containing only said call frequency, without containing any frequency of a subcarrier oscillation of said address message, means for analyzing and means for recording the received messages following said call message, retarding means connected to said means identifying the call signal and to said analyzing and recording means, and adapted to maintain said analyzing and recording means in operative condition during a determined time interval after receiving said validating signal, said analyzing means comprising means for isolating each subcarrier oscillation appearing in the messages received from said first installation, discriminating means, connected to said isolating means and adapted to detect the value of the frequency of each subcarrier frequency contained in each sequence of a message received from said first installation, said value being representative of the value of an element of information, said discriminating means being con nected to said recording means, means for decoding the serial number of each sequence of the call message connected to said recording means, means for sampling the value of an element of in formation contained in each sequence of a message received from said first installation, adapted to carry out said sampling operation at a fixed instant after the beginning of the reception of said sequence, said sampling means cooperating with said discriminating means and being connected to said recording means, safety means connected to said discriminating means and adapted to transmit the sampled value to said recording means only after having verified that in each received message each of said subcarrier frequencies is present and that only one coded value is to be found for each subcarrier oscillation, means for decoding the address, connected to said retarding means and to said recording means, adapted to produce, exclusively at the receiving of an address message which is characteristic for said second installation, an energizing signal and a signal blocking said retarding means in a position placing in operative conditions said analyzing and recording means, means for energizing said second installation, connected to said means decoding the address, and adapted to be actuated by said energizing signal, and transmission means connected to said energizing means and adapted to be actuated through said energizing signal to transmit said ready-to-operate response message corresponding to said second installation and addressed to said first installation.

26. An apparatus according to claim 25, wherein said second installation includes means for returning said second installation to a condition of reduced activity, said last mentioned means being connected to said retarding means, to said recording means and to said transmitting means and adapted to produce, at the reception of a message for returning the installation to a condition of reduced activity, a signal for releasing said retarding means and a signal for producing the transmission of a message indicating the return of said second installation to its condition of reduced activity.

27. An apparatus according to claim 25, wherein said second installation and being means for returning automatically said second installation to a condition of reduced activity and being, connected to said recorded means and to said means for sampling the value of an element of information contained in each sequence of a received message and adapted to deliver to said recording means a message for return to said condition of reduced activity, at a determined time interval from the instant of said sampling, said automatic means being adapted to return to their initial condition at the instant of said sampling exclusively upon receiving of a sequence of information belonging to a new message verified through said analyzing means, when this message is received before said time interval has elapsed.

28. An apparatus according to claim 25, wherein said transmitting means at saidsecond installation is con' nected to means determining the position of at least one regulating member of said second installation, said transmitting means being adapted to transmit, after said ready-to-operate response message, a message characterizing the position of each regulating member of said second installation.

29. An apparatus according to claim 25, wherein said first installation includes means for transmitting messages containing instructions for said second instal lation, said messages of instruction including at least one sequence containing at least one element of information numerically coded, by the value of the frequency of said subcarrier oscillation amplitude modulating said carrier wave, said second installation including means for controlling regulating members of said second installation, said controlling means being connected to said means for recording messages emanating from said first installation and to said energization means and adapted to deliver control signals for said regulating members, in response to said messages, exclusively when a signal produced by said energization means, at the receiving of an energizing signal, and a message of instructions emanating from said first installation and transmitted from said recording means are simultaneously received.

30. An apparatus according to claim 25 wherein said transmitting means at said second installation are connected to means determining the position of at least one regulating member of said installation, including means for decoding an interrogation message emanating from said first installation, said decoding means being connected to said transmitting means and to said 

1. A process for transmitting messages consisting of signals between two installations comprising the combination of the following steps: a. transmitting from one of said installations messages comprising at least one sequence of signals containing at least one element of information, the element of information being numerically coded through the frequency of at least one subcarrier oscillation which amplitude modulates a carrier wave, the frequency having a coded value selected from at least two values close to a basic frequency of said subcarrier oscillation, and b. determining at the other installation the value of the frequency of said subcarrier frequency, the determined frequency being representative of the numerical value of said element of information.
 1. A process for transmitting messages consisting of signals between two installations comprising the combination of the following steps: a. transmitting from one of said installations messages comprising at least one sequence of signals containing at least one element of information, the element of information being numerically coded through the frequency of at least one subcarrier oscillation which amplitude modulates a carrier wave, the frequency having a coded value selected from at least two values close to a basic frequency of said subcarrier oscillation, and b. determining at the other installation the value of the frequency of said subcarrier frequency, the determined frequency being representative of the numerical value of said element of information.
 2. A process according to claim 1, wherein said sequence of signals is transmitted over a period of at least one second and the signals contained in said sequence are detected at the other installation exclusively after establishment of stationary conditions of reception of said sequence.
 3. A process according to claim 1, wherein each sequence of signals includes an element of information representative of a serial number of said sequence in the transmitted message.
 4. A process according to claim 1, wherein the transmission of said messages between said installations is effected through at least one transmission cable.
 5. A process according to claim 1, wherein said messages are transmitted between said installations by means of electromagnetic waves.
 6. A process according to claim 1, wherein said messages are transmitted by means of acoustic waves.
 7. A process according to claim 1, wherein said messages are made distinctive according to their nature by the number of sequences assigned to each message.
 8. A process according to claim 1, whereiN one of said installations effects the setting into a state of reception of another installation by transmitting a call message having a single frequency, this signal constituting a carrier wave and said signal is identified at said other installation by the presence of said single frequency exclusively in said signal.
 9. A process according to claim 8, wherein at the receiving of said call message said other installation is kept in a state of reception during a determined time interval for analyzing a message following said call message.
 10. A process according to claim 1, wherein for the transmission of messages between a master station and a plurality of secondary installations, the master station places at least one secondary installation into a stage of reception by transmitting a call signal thereto and following said call signal, the master station transmits sequences of coded informations, the group of said sequences constituting an address characteristic of said secondary installation, the informations contained in the different sequences being represented by a group of different modulating subcarrier frequencies, said sequences being decoded at said secondary installations and wherein is set in condition of conversation with the master installation exclusively, the secondary installation having an address which corresponds to said address following said call signal.
 11. A process according to claim 1, wherein said sequences are constituted by a group of different basic subcarrier frequencies, wherein at least one of said subcarrier frequencies represents, according to a binary code, the value of an information, and at least one other of said subcarrier frequencies represents a serial number of said sequence, so that two sequences differ at least by the value of at least one of said subcarrier frequencies.
 12. A process according to claim 11, wherein at the reception each of said sequences is validated only after the presence of all of said subcarrier frequencies has been ascertained and after the element of information represented by each of said subcarrier frequencies has been verified, and a sampling of the signals contained in said sequence is effected after validation of said sequence.
 13. A process according to claim 10, wherein a ''''ready-to-operate'''' response-message is transmitted from said secondary installation, in response to the setting of said secondary installation into a condition of conversation, said response-message comprising the address of said installation.
 14. A process according to claim 13 wherein after reception of said response-message at said master installation, said master installation transmits coded instructions representing operations to be carried out on at least one regulating member of said secondary installation, said instructions being transmitted to means for actuating said regulating member arranged on said secondary installation, exclusively when said secondary installation is in a condition of conversation, the instruction being coded with the frequency of the carrier wave and at the same time with all the different subcarrier frequencies of said group of subcarrier frequencies.
 15. A process according to claim 14, wherein at the reception of an instruction a response-message is transmitted from said secondary installation, said response message being characteristic for the received instruction.
 16. A process according to claim 10, wherein at the setting of said secondary installation in condition of reception a device is actuated for automatically resetting said secondary installation into a condition of reduced activity when a determined time has elapsed after the reception of each sequence of a received message other than an instruction from the master station of returning to the condition of reduced activity, and wherein said automatic device is set back to its initial position exclusively upon receiving of a new message of the conversation when this message is received during said time interval.
 17. A device for transmitting messages consisting of signals between two installations, comprising in combination means for transmitting, from one of said installations, messages comprising at least one sequence of signals which contains at least one element of information, means for producing a carrier wave, means for producing at least one subcarrier oscillation, means for amplitude modulation of said carrier wave by said subcarrier oscillation, means for numerically coding said element of information, adapted to give the frequency of said subcarrier oscillation a coded frequency selected from at least two values close to a basic frequency of said subcarrier frequency and means determining at the other installation the value of the frequency of said subcarrier frequency, said frequency being representative of the numerical value of said element of information.
 18. An apparatus according to claim 17 comprising at a first of said installations, means for transmitting messages and at a second installation, means for receiving these messages, said receiving means including a retarding device adapted to place said second installation in a state of reception during a fixed time interval, at the receiving of a characteristic call message, said time interval being at least equal to the duration of a message of information to be transmitted to said second installation after said call message, means at said second installation for validating said message of information, said validating means being adapted to keep said second installation in a condition of conversation with said first installation by blocking said retarding device exclusively when said message of information is validated.
 19. An apparatus according to claim 18, wherein said second installation includes means for placing automatically said second installation back into a condition of reduced activity at the end of a predetermined time interval after the receiving of each message of information and in the absence of the reception of new message of information and in the absence of the reception of new messages emanating from said first installation.
 20. An apparatus according to claim 17, wherein each of said installations includes means for transmitting sequences of acoustic signals, said sequences having a duration greater than one second and means, in combination with said transmitting means, for receiving said sequences of signals. condition of reception of said sequence has been established.
 21. An apparatus according to claim 17 wherein at said second installation includes means for controlling the transmission of a response-message containing the address of said second installation and addressed to said first installation.
 22. An apparatus according to claim 18, wherein said first installation comprises means for displaying the informations received from said second installation, in combination with means adapted to validate the sequences of said message containing informations, prior to the displaying of these informations.
 23. An apparatus according to claim 17, wherein at said second installation includes means for sampling signals of said sequences.
 24. An apparatus according to claim 23, wherein said second installation includes means for blocking the operation of said sampling means during the transmission periods of said second installation and during a fixed time interval following each transmission period.
 25. An apparatus for transmitting from a first installation messages destined to a second installation which is in a state of reduced activity, said messages comprising call and address messages for said second installation, so as to place in state of reception and energize said second installation at the receiving of said call and address messages and for transmitting from said second installation, in response to a call and address message a ''''ready-to-operate'''' response-message destined to said first installation, this apparatus comprising in combination: a. at said first instAllation, means for transmitting a call message constituted by a signal at a call frequency, means for transmitting messages including at least an address message corresponding to said second installation and following said call message, said address message being constituted by at least one sequence of informations coded numerically by the value of the frequency of at least one subcarrier oscillation amplitude modulating a carrier wave, this coding frequency being selected between at least two values close to a basic frequency of said subcarrier oscillation, each sequence of the address message also including at least one information signal representative of a serial number of said sequence in said address message, and means for receiving a ''''ready-to-operate'''' response-message transmitted from said second installation, in response to said address message; b. at said second installation, means for receiving messages emanating from said first installation and means for identifying said call message, adapted to deliver a validating signal at the receiving of a message containing only said call frequency, without containing any frequency of a subcarrier oscillation of said address message, means for analyzing and means for recording the received messages following said call message, retarding means connected to said means identifying the call signal and to said analyzing and recording means, and adapted to maintain said analyzing and recording means in operative condition during a determined time interval after receiving said validating signal, said analyzing means comprising means for isolating each subcarrier oscillation appearing in the messages received from said first installation, discriminating means, connected to said isolating means and adapted to detect the value of the frequency of each subcarrier frequency contained in each sequence of a message received from said first installation, said value being representative of the value of an element of information, said discriminating means being connected to said recording means, means for decoding the serial number of each sequence of the call message connected to said recording means, means for sampling the value of an element of information contained in each sequence of a message received from said first installation, adapted to carry out said sampling operation at a fixed instant after the beginning of the reception of said sequence, said sampling means cooperating with said discriminating means and being connected to said recording means, safety means connected to said discriminating means and adapted to transmit the sampled value to said recording means only after having verified that in each received message each of said subcarrier frequencies is present and that only one coded value is to be found for each subcarrier oscillation, means for decoding the address, connected to said retarding means and to said recording means, adapted to produce, exclusively at the receiving of an address message which is characteristic for said second installation, an energizing signal and a signal blocking said retarding means in a position placing in operative conditions said analyzing and recording means, means for energizing said second installation, connected to said means decoding the address, and adapted to be actuated by said energizing signal, and transmission means connected to said energizing means and adapted to be actuated through said energizing signal to transmit said ''''ready-to-operate'''' response message corresponding to said second installation and addressed to said first installation.
 26. An apparatus according to claim 25, wherein said second installation includes means for returning said second installation to a condition of reduced activity, said last mentioned means being connected to said retarding means, to said recording means and to said transmitting means and adapted to produce, at the reception of a message for returning the installation to a condition of reduced activity, a signal for releasiNg said retarding means and a signal for producing the transmission of a message indicating the return of said second installation to its condition of reduced activity.
 27. An apparatus according to claim 25, wherein said second installation and being means for returning automatically said second installation to a condition of reduced activity and being, connected to said recorded means and to said means for sampling the value of an element of information contained in each sequence of a received message and adapted to deliver to said recording means a message for return to said condition of reduced activity, at a determined time interval from the instant of said sampling, said automatic means being adapted to return to their initial condition at the instant of said sampling exclusively upon receiving of a sequence of information belonging to a new message verified through said analyzing means, when this message is received before said time interval has elapsed.
 28. An apparatus according to claim 25, wherein said transmitting means at said second installation is connected to means determining the position of at least one regulating member of said second installation, said transmitting means being adapted to transmit, after said ''''ready-to-operate'''' response message, a message characterizing the position of each regulating member of said second installation.
 29. An apparatus according to claim 25, wherein said first installation includes means for transmitting messages containing instructions for said second installation, said messages of instruction including at least one sequence containing at least one element of information numerically coded, by the value of the frequency of said subcarrier oscillation amplitude modulating said carrier wave, said second installation including means for controlling regulating members of said second installation, said controlling means being connected to said means for recording messages emanating from said first installation and to said energization means and adapted to deliver control signals for said regulating members, in response to said messages, exclusively when a signal produced by said energization means, at the receiving of an energizing signal, and a message of instructions emanating from said first installation and transmitted from said recording means are simultaneously received. 